1. Field of the Invention
The present invention relates to an exhaust flow rate control valve provided in an exhaust pipe through which exhaust gas of an engine is passed.
2. Description of the Background Art
There is a conventionally known exhaust flow rate control valve 100 (FIG. 6) including: an on-off valve 105 which is provided in an exhaust pipe through which exhaust gas of an engine is passed and which is switched between a closed state in which a flow passage sectional area in the exhaust pipe is decreased and an opened state in which the flow passage sectional area is increased; and a resilient member 126 for biasing the on-off valve 105 in a closing direction, the on-off valve 105 being switched from the closed state to the opened state by an increase in pressure of the exhaust gas acting on the on-off valve 105 against a biasing force of the resilient member 126 (see U.S. Pat. No. 5,355,673).
In the exhaust flow rate control valve disclosed in U.S. Pat. No. 5,355,673, the resilient member 126 and a link mechanism 106, which is interposed between the resilient member 126 and the on-off valve 105, are disposed between the exhaust pipe and an outer pipe 107 which surrounds the exhaust pipe, thereby preventing the resilient member 126 and the link mechanism 106 from being thermally deteriorated due to their direct exposure to the exhaust gas or damaged due to flying stones and the like. Further, the resilient member 126 and the link mechanism 106 are formed to be thin in a radial direction of the pipe, and compactly accommodated between the exhaust pipe and the outer pipe 107.
However, depending on the setting of the operating characteristic of the on-off valve 105, the resilient member 126 and the link mechanism 106 cannot be always formed to be thin. In this case, it is necessary to increase the diameter of the outer pipe 107 to radially enlarge the space formed between the exhaust pipe and the outer pipe 107 so that the resilient member 126 and the link mechanism 106 can be accommodated therein. However, this arrangement is problematic since the exhaust structure is increased in size at a place where the exhaust flow rate control valve is provided.